This invention pertains to rotary motion feedthrough devices which are sealed by magnetic fluid ("ferrofluid"). Such devices commonly employ a magnetic pole piece assembly to provide suitable magnetic flux in a set of annular gaps disposed axially along a rotating shaft.
FIGS. 1A and 1B taken from U.S. patent application Ser. No. 08/940,777 referenced above show a typical example of a rotary motion feedthrough structure 100 of the prior art. Five pole piece rings 20 are arranged in a stack with four ring magnets 18 to form a pole piece assembly 16. The entire assembly is mounted within a housing 10, which also supports a shaft 14 and a bearing 12 assembly.
Shaft 14 has an outside diameter slightly smaller than the inside diameter of pole rings 20, so a small annular gap 22 exists between each pole ring and the shaft. This gap is typically 0.002 inch in radial dimension. Ferrofluid fills each gap, being held in place by magnetic forces.
A sealing material (not shown) fills empty spaces 21 (FIG. 3) between the magnets 18 and pole rings 20, preventing leakage from the outer diameter of the pole rings radially inward to the ferrofluid sealing region. It is necessary to provide static sealing (as will be described below) at every one of the eight interfaces between pole rings 20 and ring magnets 18. If this is not done, each of the seals made by the eight fluid rings would be bypassed by gas leaking across the pole ring/magnet interfaces. This would result in the full pressure differential (typically 1 atmosphere) appearing across the final fluid ring at the left end of the pole piece. Since it is not possible to support this much pressure difference across a single fluid ring, the seal would fail. If carefully formulated and applied, the sealing material also serves to provide mechanical retention of the magnets 18 in their proper locations. A single O-ring seal 30 provides static sealing between the pole piece assembly 16 and housing 10 at the vacuum side of the pole piece.
The five pole piece rings 20 must be precisely aligned (typically within 0.0005") with each other and with the axis of the rotating shaft in order to produce eight annular gaps 22 which can be filled with ferrofluid. This alignment is accomplished during assembly of the pole piece by mounting the pole rings 20 on a fixturing shaft (not shown) having a diameter which matches the inner diameter (ID) of the pole rings very closely (typically within 0.0002"). The stack of pole rings and magnets is then held on the fixture and a static sealing material (typically epoxy resin and hardener) is applied and allowed to cure. Curing time is usually several hours.
FIG. 2 is an isometric view of a typical single pole ring 20 of the prior art with a circular array of short cylindrical magnets 18A placed on one surface. Although a single ring magnet could be used, an array of small magnets is often used instead, because many different seal sizes can be made using only one or two types of standardized small magnets, thereby simplifying production planning and inventory control. Typical magnet dimensions are 4.5 mm or 9 mm diameter and 2 mm high. Enough magnets are placed in each layer to occupy substantially the entire space available. It is clear from FIG. 2 that a lot of empty space must be filled with sealing material.
Close examination of FIG. 2 also reveals that a small raised rim 15 exists at the outer diameter of the pole ring. Each magnet has been placed so that it abuts the inner diameter of this rim. The rim is required because the magnets exert mutually repulsive forces on each other, tending to push all magnets radially away from the axis of the pole ring. This force becomes particularly significant as the last magnet is placed on the ring. If there were no retaining rim, one or more magnets might move radially outward and protrude beyond the outer diameter of the pole ring.
FIG. 3 shows a stack of four pole rings 18A and their associated magnet layers in a complete pole piece assembly. The topmost pole ring has been omitted for clarity.